1. Field of the Invention
The present invention relates to a high wear-resistant bearing material to be used under a humid environment, and particularly to a bearing material used in artificial joint as a prosthesis for a joint of human body.
2. Description of the Related Art
Ultra-high molecular weight polyethylene (hereafter referred to as UHMWPE) is used as a common material to form a bearing material that constitutes the sliding surface of an artificial knee joint such as artificial hip joint or artificial knee joint. However, wear debris of UHMWPE generated by frictional motion of the artificial joint used in the human body has a potential of inducing osteolysis. A decrease in the bonding force between the artificial joint and bone, the so-called loosening, caused by osteolysis has been a focus of great concern as a complication of joint arthroplasty. Linear wear of UHMWPE is usually from about 0.1 mm to 0.2 mm annually, which does not pose a problem for some time after undergoing the joint arthroplasty, although the loosening becomes significant after a period of about five years. Then the artificial joint needs to be replaced, thus placing a great burden on the patient.
One measure to avoid the loosening is to reduce the amount of the wear debris generated from the UHMWPE. For this purpose, various attempts have been made such as seeking better combination of materials to make the joint surface and improvement of the material itself. These attempts include the vigorous researches conducted on UHMWPE of which molecular chain is crosslinked through irradiation with electron beam or gamma ray (crosslinked polyethylene, hereafter referred to as CLPE) (for example, specification of Japanese Patent No. 2,984,203, specification of U.S. Pat. No. 6,228,900 and International Publication No. WO 97/29793, pamphlet). These researches are based on the fact that irradiation of a polymer material with high energy beams such as electron beam or gamma-ray generates free radicals due to scission of molecular chains, followed by recombination of a molecular chain and a crosslinking reaction. The CLPE has higher wear resistance than the conventional UHMWPE, and it has been reported that the wear can be decreased to one fifth or as small as one tenth that of the conventional material.
Many research efforts have also been directed to the improvement of tribological characteristic of the surface of the bearing material through the formation of a film layer on the UHMWPE. For example, such a technique has been known that a film of a random copolymer having allylamine and a phosphorylcholine analogous group is fixed onto the material surface used for a medical purpose that is required to have excellent tribological characteristic such as artificial joint, so as to render biocompatibility and surface lubrication (for example, International Publication No. WO 01/05855, pamphlet).
Particularly high effect of suppressing wear of the artificial joint can be achieved by grafting a polymerizable monomer having a phosphorylcholine group onto the bearing surface of an artificial joint made of UHMWPE, thus providing an artificial joint component made of a polymer material that has a remarkable effect of suppressing wear of the artificial joint and is capable of suppressing the generation of wear debris (for example, Japanese Patent Unexamined Publication (Kokai) No. 2003-310649).
Even when the bearing surface of a joint replacement is modified to make it resistant to wear, the technologies currently available are not capable of completely prevent wear of the bearing surface. Thus the bearing material of a joint replacement with the surface modified by the prior art technology may demonstrate high wear resistance in the initial stage of use, but would show rapidly deteriorating wear resistance once the modified surface region has been removed by wear or delamination over a long period of use. Then, if the decreasing wear resistance leads to the generation of wear debris, the possibility is high for loosening to take place.
The bearing material of a joint replacement made by using the CLPE has a short history of clinical application, and therefore it has not been verified whether its wear resistance would be maintained over an extended period of time.
In case the technique of fixing a film of a random copolymer onto the material surface used for a medical purpose is applied to a bearing material of a joint replacement, the film made of a random copolymer is subjected to severe friction that causes wear and is quite likely to remove the UHMWPE surface. Thus it is difficult to apply this technology into practical use.
Such a remove occurs because of weak bonding strength between the UHMWPE surface and the random copolymer layer. The bonding strength is weak partly because the random copolymer layer in which polymerization has progressed to a significant level is fixed onto the UHMWPE surface, and partly because of the absence on the UHMWPE surface of a functional group that enables it to bond with the polymerized random copolymer layer.
In contrast, Japanese Patent Unexamined Publication (Kokai) No. 2003-310649 describes a technology that successfully increased the bonding strength between a polymer chain having a phosphorylcholine group and UHMWPE surface by grafting a polymer chain having a phosphorylcholine group and the surface of a UHMWPE substrate. This enabled it to obtain a bearing material of a joint replacement having sliding surface made of UHMWPE whereon a high wear-resistant film, that does not remove even when used under harsh friction and wear conditions, is formed. However, it has not been verified whether the wear resistance of this bearing material of a joint replacement would be maintained over an extended period of time. For example, Japanese Patent Unexamined Publication (Kokai) No. 2003-310649 describes an accelerated joint simulator wear test in which a stainless steel ball against the bearing material of a joint replacement was performed up to three million cycles, which is equivalent to three years of use and does not prove the high wear-resistance during use over the lifetime typically required of an artificial joint (for example, five years).